Joined: 15 Dec 2002 Posts: 637 Topics: 43 Location: Bengaluru, INDIA
Posted: Mon Apr 21, 2003 6:00 am Post subject: PHYREC/TRK and PHYREC-SIZE
Hi,
I am a beginner in VSAM. I tried some DEL/DEF/LISTC jobs to understand the VSAM Space allocation. I am not able to interpret the LISTCAT listing correctly. The two parameters in particular are, PHYREC-SIZE and PHYRECS/TRK. I looked up their meaning in the manual. The manual says:
Quote:
PHYREC-SIZE
The number of bytes that VSAM uses for a physical record in the data or index component.
Quote:
PHYREC/TRK
The number of physical records (of the size indicated under PHYRECS-SIZE) that VSAM can write on a track on the volume.
My questions are:
1. If I have a KSDS defined for a RECORDSIZE as RECORDSIZE(80 400), then will VSAM use PHYREC-SIZE number of bytes for any record with length between 80 and 400 (limits included)? (Somehow, I do not like this statement.) If not, then what does this mean? Isn't it the CISIZE itself?
2. The parameter, CI/CA means (from the manual),
Quote:
CI/CA represents the number of control intervals per control area.
Since, maximum size of a CA is a cylinder, I thought that CI/CA is actually number of CI per 15 tracks (15 tracks make a cylinder). I was justified because, PHYREC/TRK was always equal to ratio of CI/CA and 15. So, isn't "...the number of physical records.... VSAM can write on a track on the volume" equal to (number of records in a CI) * (PHYREC/TRK)?
3. In some of the jobs that I submitted, I had coded CISZ as 14336 but the PHYREC-SIZE was 7168. What makes VSAM to code such a low value? _________________ ALL opinions are welcome.
Debugging tip:
When you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth.
-- Sherlock Holmes.
Joined: 02 Dec 2002 Posts: 22 Topics: 0 Location: Sydney, Australia
Posted: Mon Apr 21, 2003 5:06 pm Post subject:
1) recordsize(80 400) allows for individual records between 80 and 400 bytes(a bit like a VB file).
2) Correct.
3) Vsam in its wisdom in conjunction with the device (in your case a 3390) PHYSICALLY stores blocks of 7168 to make up a CISZ of 14336.
I don't know your application but I would use CISZ of 4096. _________________ Rainer
Joined: 15 Dec 2002 Posts: 637 Topics: 43 Location: Bengaluru, INDIA
Posted: Mon Apr 21, 2003 10:48 pm Post subject:
R.Nickel,
Rainer, thanks for your responses.
1)I know that, with the above RECORDSIZE parameter, "variable" records will be written. The manual says, "The number of bytes that VSAM uses for a physical record in the data or index component." for PHYREC-SIZE. My doubt is, whether PHYREC-SIZE is the number of bytes that VSAM uses for any physical record. Also, isn't the PHYREC-SIZE same as the CISIZE itself?
3) If the PHYREC-SIZE is the size of a block that VSAM uses to make up a CI, then is the CISZ still the one that I code in my JCL?
Rainer, I am not designing any application. It was just my curiosity that made me post here.
Just curious.....Why is 4096 bytes the favorite CISZ? _________________ ALL opinions are welcome.
Debugging tip:
When you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth.
-- Sherlock Holmes.
Joined: 02 Dec 2002 Posts: 22 Topics: 0 Location: Sydney, Australia
Posted: Mon Apr 21, 2003 11:18 pm Post subject:
PHYREC-SIZE is not CISZ, it is a result caused by VSAMs' CISZ and the architecture of the device type, you only need to refer to the CISZ.
CISZ of 6656 on a 3350 would create PHYREC-SIZE of 512, 72% of each track would be usable.
However, with the use of the next larger CISZ 7168 PHYREC-SIZE would be 3584 and 93% utilization.
Next smaller CISZ 6144 PHYREC-SIZE would be the same as the CISZ 6144 and utilization of 96%.
All these examples refer to 3350 device and DATA portion of the Vsam file, other device types and INDEX calculations have their own idiosyncrasies.
A CISZ of 4096 is a good starting point when creating a VSAM file, it can be accessed by both online (not too much data is returned in a random read) & batch (easy to buffer for sequential reads) PHYREC-SIZE is the same as CISZ for all device types, and has consistent good utilization across device types. _________________ Rainer
Joined: 15 Dec 2002 Posts: 637 Topics: 43 Location: Bengaluru, INDIA
Posted: Tue Apr 22, 2003 12:03 am Post subject:
R.Nickel,
Thank you for the responses.
Suppose, I want to calculate the space for a KSDS cluster where average and max record length is 80. I need to know how many records are there in a CI. Using this, I can arrive at the number of cylinders required for, say, 1000000 records. To know the number of records per CI, I need to divide the CISZ by (LRECL +10). So, I thought, I would divide the CISZ which I code in my JCL. Then, I took a closer look in the LISTCAT listing and found the differences between PHYREC-SIZE and the CISZ that I coded.
So, the main reason for posting this query is to know what should I use for the numerator when I want to find the number of records per CI. The CISZ that I code in the JCL? Or, the PHYREC-SIZE that appears in the CISZ?
I am sorry that I did not make my intentions clear at the outset. But, I thought, I would be able to figure out myself once I know more about PHYREC-SIZE and PHYRECS/TRK. _________________ ALL opinions are welcome.
Debugging tip:
When you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth.
-- Sherlock Holmes.
Joined: 02 Dec 2002 Posts: 22 Topics: 0 Location: Sydney, Australia
Posted: Tue Apr 22, 2003 12:28 am Post subject:
Quote:
PHYREC-SIZE is not CISZ, it is a result caused by VSAMs' CISZ and the architecture of the device type, you only need to refer to the CISZ.
number of records per CISZ = TRUNC(CISZ/LRECL)
number of CIs per CA = depends on the device type and the CISZ that you select, for 4096 on a 3390 with no free space the answer is 180 or 168 if you use the IMBED option for the INDEX.
in cylinders, 1,000,000 80 byte records, no freespace, noimbed, 3390 device and CISZ 4096 = 109 cylinders _________________ Rainer
You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum
FAQ
Search
Quick Manuals
Register
Profile
Log in to check your private messages
Log in
